This invention relates to bearing assemblies.
Diamond-bearing assemblies are known and are used in various applications. One application of a diamond-bearing assembly is a downhole motor used in the drilling industry.
In diamond-bearing assemblies, the opposed bearing surfaces of the assembly are both made of a diamond material. The diamond material is polycrystalline diamond or PCD. This diamond material is also known in the art as a diamond compact. Such material is used because of its long wearing life and its ability to carry substantial load. The polycrystalline diamond which has been used for diamond-bearing assemblies is that known in the art as Syndite.RTM., Syndrill.RTM. or Stratapax.RTM.. This polycrystalline diamond material contains substantial amounts of Group VIII metals such as cobalt, iron or nickel. Reference in this regard may be made to U.S. Pat. No. 4,620,601, column 12 lines 30 to 37, and U.S. Pat. No. 4,560,014, column 9 lines 29 to 44.
U.S. Pat. 5,092,687 describes diamond-bearing assemblies having opposed diamond-bearing surfaces. The diamond-bearing surfaces are provided by cemented tungsten carbide in which synthetic diamonds are bonded--column 3 lines 49 to 51. Cemented tungsten carbide has a Group VIII metal such as cobalt, nickel or iron as its binder.
It has been found that where a diamond-bearing assembly utilises a diamond material containing a Group VIII metal, as in the references described above, the friction behaviour of the contacting-bearing surfaces tends to become quite unpredictable during high contact pressures. In some cases, rapid seizure: of the bearing assembly occurs without adequate warning. This phenomenon is believed to be caused by a Group VIII-rich tribofilm which forms on the diamond surfaces during use of the bearing assembly.